1. Field of the Invention
The present invention relates to a continuous electrodeionization apparatus and method and, more particularly, to a continuous electrodeionization apparatus and method that provide improved removal of weakly ionized ions such as silica, and enhanced overall performance.
2. Description of the Related Art
Electrodeionization (EDI) is a process that removes ionizable species from liquids using electrically active media and an electrical potential to influence ion transport. The electrically active media may function to alternately collect and discharge ionizable species, or to facilitate the transport of ions continuously by ionic or electronic substitution mechanisms. EDI devices include media of permanent or temporary charge, and may be operated to cause electrochemical reactions specifically designed to achieve or enhance performance. These devices also include electrically active membranes such as semipermeable ion exchange or bipolar membranes.
Continuous electrodeionization (CEDI) is a process wherein the primary sizing parameter is the transport through the media, not the ionic capacity of the media. A typical CEDI device includes alternating electroactive semipermeable anion and cation exchange membranes. The spaces between the membranes are configured to create liquid flow compartments with inlets and outlets. A transverse DC electrical field is imposed by an external power source using electrodes at the bounds of the membranes and compartments. Often, electrolyte compartments are provided so that reaction product from the electrodes can be separated from the other flow compartments. Upon imposition of the electric field, ions in the liquid are attracted to their respective counterelectrodes. The compartments bounded by the electroactive anion permeable membrane facing the anode and the electroactive cation membrane facing the cathode become ionically depleted, and the compartments bounded by the electroactive cation permeable membrane facing the cathode and the electroactive anion membrane facing the anode become ionically concentrated. The volume within the ion-depleting compartments and, in some embodiments, within the ion-concentrating compartments, also includes electrically active media. In CEDI devices, the media may include intimately mixed anion and cation exchange resins. The ion-exchange media enhances the transport of ions within the compartments and can also participate as a substrate for controlled electrochemical reactions.
The removal of weakly ionized ions, such as silica, using electrodeionization has been the subject of much research. Compared with other materials commonly found in water, silica is typically present only in trace amounts. However, its removal is important in the production of high purity water, wherein every trace constituent present in the feed water must be removed. It is well known that systems such as electrodialysis do not remove silica and that electrodeionization and electroregeneration techniques do not completely remove silica. The inability to adequately remove silica from a feed water stream thus greatly reduces the applicability of the above techniques in high purity applications, including the largest high purity application, boiler feed water.
Silica is weakly ionized and is not transported efficiently by ion exchange resins within the compartments or through the membranes of an electrodeionization device. It has been found that resins having a substantially uniform bead diameter allow substantially complete removal of weakly ionized carbonic acid from a feed stream, but do not improve silica removal to the same extent. However, when CEDI is operated at high voltage and/or low flow rates, the resin can pick up silica. The silica must be in ionic form on the resin and must therefore have transferred successfully through what is known in the art as the "film boundary layer barrier." Even in these cases, however, total transfer does not occur.
Researchers have suggested pH adjustments of the water to a more basic form to ionize silica and enhance its removal from feed water. However, such pH adjustments have been found to have only a moderate affect on silica removal. Instead of removing silica, the equipment rapidly removes the hydroxide ion that was added during pH adjustment and leaves the silica behind. In addition, attempts to remove silica by bipolar electroregeneration of resin have resulted in incomplete removal.
Ganzi et al., in U.S. Pat. No. 5,316,637, disclose an apparatus and method for removing weakly ionized ions from feed water. It is generally thought that the electrochemical removal of the weakly ionized silica requires an anion exchange resin material that contains a relatively low degree of crosslinking and/or relatively high water content, as well as a substantially uniform bead size. However, these materials tend to have expansion, small bead size, high-pressure loss, and reduced chemical and oxidation resistance.
Highly crosslinked resins typically provide poor silica removal because they are poor in transporting silica. Macroporous ion exchange resins are typically relatively highly crosslinked and typically contain relatively low water content. However, the characteristics of high crosslinking and low water content contribute to high electrical resistivity in a CEDI apparatus, especially in high purity water applications.